Desalination systems are systems that remove salt or other dissolved solids from water, most often to produce potable water. Currently, several methods of desalination are employed by commercial desalination systems. The most popular methods of commercial desalination are reverse osmosis and flash vaporization. Both of these methods have large energy requirements and certain components that wear out frequently. For example, reverse osmosis systems force water through membranes and these membranes become clogged and torn, thus necessitating frequent replacement. Similarly, flash vaporization systems have corrosion and erosion problems due to the spraying of hot brine within these systems. The energy requirements for a reverse osmosis system may be approximately 6 kWh of electricity per cubic meter of water, while a flash vaporization system may require as much as 200 kWh per cubic meter of water. Due to the high energy inputs and frequent maintenance, desalination of water on a large scale basis has been relatively expensive, often more expensive than finding alternate sources of groundwater.
Submerged gas evaporator systems in which gas is dispersed into a continuous liquid phase, referred to generally herein as submerged gas evaporators, are well known types of devices used to perform evaporation processes with respect to various constituents. U.S. Pat. No. 5,342,482, the entire specification of which is hereby incorporated by reference, discloses a common type of submerged combustion gas evaporator, in which combustion gas is generated and delivered though an inlet pipe to a dispersal unit submerged within the liquid to be evaporated. The dispersal unit includes a number of spaced-apart gas delivery pipes extending radially outward from the inlet pipe, each of the gas delivery pipes having small holes spaced apart at various locations on the surface of the gas delivery pipe to disperse the combustion gas as small bubbles as uniformly as practical across the cross-sectional area of the liquid held within the processing vessel. According to current understanding within the prior art, this design provides desirable intimate contact between the liquid and the combustion gas over a large interfacial surface area while also promoting thorough agitation of the liquid within the processing vessel.
Because submerged gas evaporators disperse gas into a continuous liquid phase, these devices provide a significant advantage when compared to conventional evaporators when contact between a liquid stream and a gas stream is desirable. In fact, submerged gas evaporators are especially advantageous when the desired result is to highly concentrate a liquid stream by means of evaporation.
However, during the evaporation process, dissolved solids within the continuous liquid phase become more concentrated often leading to the formation of precipitates that are difficult to handle. These precipitates may include substances that form deposits on the solid surfaces of heat exchangers within flash vaporization systems or on the membranes of reverse osmosis systems, and substances that tend to form large crystals or agglomerates that can block passages within processing equipment, such as the gas exit holes in the system described in U.S. Pat. No. 5,342,482. Generally speaking, feed streams that cause deposits to form on surfaces and create blockages within process equipment are called fouling fluids.
Deposits of precipitated solids create chemical fouling or buildup on fill or packing within conventional desalination systems that increases available surface area and also create stagnant flow areas that leads to biological fouling of these surfaces by promoting growth of bacteria and algae. Biological growth leads to the formation of slime within a desalination system that further reduces desalination efficiency and can also foul heat exchangers within equipment which employs the circulating liquid from the desalination system as an evaporative medium
These common problems adversely affect the efficiency and costs of conventional desalination systems in that they necessitate frequent cleaning cycles and/or the addition of chemical control agents to the evaporative fluid to avoid loss of efficiency and to avoid sudden failures within the evaporation equipment.
Additionally, most evaporation systems that rely on intimate contact between gases and liquids are prone to problems related to carryover of entrained liquid droplets that form as the vapor phase disengages from the liquid phase. For this reason, most evaporator systems that require intimate contact of gas with liquid include one or more devices to minimize entrainment of liquid droplets and/or to capture entrained liquid droplets while allowing for separation of the entrained liquid droplets from the exhaust gas flowing out of the evaporation zone. Droplets within the vapor are particularly troublesome if the process is applied to produce potable water in that the entrained droplets contain the salts, minerals and other contaminants that were in the feed liquid.
Unlike conventional evaporators where heat and mass are transferred from the liquid phase as it flows over the extended surface of the heat exchangers, heat and mass transfer within submerged gas processors take place at the interface of a discontinuous gas phase dispersed within a continuous liquid phase and there are no solid surfaces upon which deposits can accumulate.
Submerged gas evaporators also tend to mitigate the formation of large crystals because dispersing the gas beneath the liquid surface promotes vigorous agitation within the evaporation vessel, which is a less desirable environment for crystal growth than a more quiescent zone. Further, active mixing within an evaporation vessel tends to maintain precipitated solids in suspension and thereby mitigates blockages that are related to settling and/or agglomeration of suspended solids.
However, mitigation of crystal growth and settlement is dependent on the degree of mixing achieved within a particular submerged gas evaporator, and not all submerged gas evaporator designs provide adequate mixing to prevent large crystal growth and related blockages. Therefore, while the dynamic renewable heat transfer surface area feature of submerged gas evaporators eliminates the potential for fouling liquids to coat extended surfaces, conventional submerged gas evaporators are still subject to potential blockages and carryover of entrained liquid within the exhaust gas flowing away from the evaporation zone.